By Marisa DiBiaso, Project Assistant
Water is the most destructive force in roads. • Triangular--The v-shaped bottom ditch is the
Ditches are the best way to carry water away from most easily constructed and requires the least
roads and to the water table. Ditches catch sedi- roadside area. Of the three types, it requires
ment carried off the road surface. This makes it more maintenance, has the lowest water-car-
imperative that road managers keep ditches in rying capacity, and is the most susceptible to
good condition. This article will cover the basics erosion.
of good ditches: inspection, shape, slope, lining,
vegetation, and maintenance and repair. Slope
Inspection The lengthwise slope affects the velocity of
flow. If the slope is too steep, the swift water flow
The first step to ditch basics is to regularly causes erosion and maintenance problems. If the
clean and inspect ditches to keep them working slope is too flat, water stays in the ditch and can
properly. Use the following checklist to help infiltrate the road base. Water in the base deterio-
identify needed maintenance. rates the road from within. To ensure water flow,
o Is the ditch free of obstructions? the minimum desired lengthwise slope is 1.0%. It
o Does the ditch have a clear outlet? should never be less than 0.5%. The maximum for
o Is the ditch deep enough to drain the subgrade an unlined ditch is 5%. If the slope exceeds 5%,
and cut off subsurface water? the ditch should be lined as described below to
o Is the ditch broad enough? prevent erosion.
o Is the slope uniform; that is, free of high or Ditches with earth sides should have side
low points to minimize ponding? slopes from 1:4 or 1:2 (drop to run). To calculate
o Could debris control devices be used? slopes, see page 9 for a copy of "Measuring and
o Is there any erosion? Calculating Slopes."
o Is a lining needed/does the existing lining need
o Is the slope adequate?
Erosion creates sediment that is deposited in
Shapes ditches and water bodies. The greater the erosion,
the more maintenance needed. Excessive erosion
Highway crews can construct and maintain can weaken the sides of the road itself.
ditches in three cross-section shapes: parabolic Ditch lining is the most common way to pre-
(round bottom), trapezoidal (flat bottom), or trian- vent erosion. Linings can be of natural soil, vege-
gular (v-shaped bottom). tative, or paved. The material used depends on
• Parabolic--This ditch is best in terms of long- flow velocity. The following table illustrates the
term cost and efficiency. It has about the same types of lining appropriate for various flow
capacity as the trapezoidal with less erosion. velocities.
Sides are easily vegetated, further reducing Engineers use similar tables when designing
erosion. It is probably the most difficult and waterways. Road managers can also use it empiri-
expensive to construct. cally. If a lined, well-maintained ditch is eroding,
• Trapezoidal--The flat bottom is easier to con- then crews should apply a lining for a higher flow.
struct than the round bottom (parabolic)' ditch. If problems persist, the agency should consult a
Compared to the triangular shape, the flat qualified engineer.
bottom slows water and reduces erosion. It
takes more time and expense to construct, but
requires less maintenance and greater capacity.
Road Business, Winter 2000, Vol. 15, No.4
Maximum Periodically maintain all ditches. Schedule
Type of Lining Flow routine maintenance" every five years. Poorly
Velocity shaped, sloped, or lined ditches require mainte-
'" Natural Soil Linings
Rip-rap sides and bottoms
nance more often. To maintain ditches in a five-
year cycle, divide municipal roads into five areas.
Plan and schedule work on each section in a given
Cleansand 1-2 Planning begins with inspection. Inspection in
Silty sand, clay 2-3 the fall enables a clear view of ditch conditions,
Clayeysand, silt 3-4 and allows planning over the winter. Planned
Vegetative Linings maintenance should produce ditches with adequate
Average turf, erosion resistant soil 4-5 shape, slope, and linings. The following guidelines
Average turf, easily eroded soil 3-4 should be considered:
Dense turf, erosion resistant soil 6-8 • The most efficient and effective ditches have
Gravel bottom, brushy sides 4-5 flat or round bottoms. A backhoe or excavator
Dense weeds 5-6 most easily constructs trapezoidal or parabolic
Paved Linings shaped ditches. If using a grader, run the wheel
Gravel bottom, concrete sides 8-10 the ditch bottom.
Mortared rip-rap 8-10
• Compact the ditch bottom.
Concrete or Asphalt 18-20
• The lengthwise and side slopes should be uni-
• Immediately seed, mulch, and use fiber mats
In addition to minimizing erosion in low-flow to establish vegetation, or immediately apply
ditches, vegetation removes roadway pollutants other necessary lining.
from runoff water. Here are a few guidelines to Ditches should be an appropriate depth. To
remember about vegetation in ditches. keep water out of the base material, the ditch bot-
• Vegetative linings should be established tom should be below the base course. A depth of
BEFORE erosion begins. 18 inches is usually sufficient, but it may have to
• Seed, mulch, and, where necessary, fiber mats be deeper if water flows in from hillsides. The
should be applied immediately after any ditch distance between cross culverts or ditch outlets
maintenance or storm damage. influence depth. Inspection will indicate if depth is
• Fertilizer often speeds growth, but excessive adequate, or 'if culverts or outlets should be
fertilization can cause groundwater and sur- installed.
face water quality problems. Ditches are often constructed too deep. Lining
• Mowing prevents weeds and woody vegeta- is more difficult, and the added depth increases
tion. Mowing too close will reduce the erosion ditch erosion. Deep ditches can create a safety
resistance ofthe lining. hazard to motorists, especially on roads with nar-
Maintenance and Repair Ditches are necessary to carry water off and
away from roads and must be regularly main-
Ditch repair should be included in all road
tained. They must have the proper shape, slope,
repair projects. Road rehabilitation and recon-
and lining. Water flow, road surface sediment, and
struction are expensive and some municipalities
growing vegetation attack these features.
omit ditch repair to save money. Some ignore
ditches in minor projects, too. Inadequate ditches Sources:
will allow water to infiltrate the road base. This Basics of a Good Road. UNH Technology Transfer Center. Durham, NH.
creates inadequate drainage resulting in faster road "Ditch/Channel Construction & Maintenance." A Series of Quick Guides for
deterioration than if the time and money was spent New Hampshire Towns. UNH Technology Transfer Center. Durham, NH.
Drainage,Drainage, Drainage. UNH Technology Transfer Center. Durham
on proper ditching. NH. January 1996.
Road Business, Winter 2000, Vol. 15, No.4